Is a rusty shovel blade worth saving or am I wasting my time?

Vinegar does work on rust but it's slow and it only really pulls off the loose surface stuff. On a shovel head that's been sitting for years I'd go mechanical first. Wire wheel on a drill eats that crust off in minutes, or a finer grit on a flap disc if you've got one. Get the heavy stuff off mechanically, then a vinegar soak overnight for what's left, scrub with a scouring pad, rinse, dry it imediately and wipe the whole head down with something like WD-40 or boiled linseed oil so flash rust doesn't come back overnight. Don't leave steel in vinegar for days, it keeps eating and you end up with a black etched mess. That dried gunk near the edge is almost certainly mortar or concrete, and vinegar won't touch cured cement. Two options there. Chisel or screwdriver it off mechanically if it comes off in chunks, or get some muriatic acid from a hardware store, brush a bit on, let it fizz, scrub, then rinse really well. It will also strip any remaining rust so do the whole head after. Do this outside, and wear gloves and eye protection since it'll burn skin and splash.

To check whether the blade is worth saving, hold it up to a light and look at the cutting edge. A few pits that feel like shallow dents are fine, they'll still dig. If the edge area is flaking, scaling, or you can feel deep craters with a screwdriver tip, or worse the steel is thinned and flexible along the lip, that head is toast. Scuffed lines across the middle don't matter at all, that's just cosmetic. On the handle, no, it's not just a loose wedge job. Replacement shovel handles come with a long tapered end that matches the socket taper. You push it in as far as it'll go by hand, then you flip the whole shovel and slam the handle down onto a hard surface a few times, hard. The taper self-tightens as it seats and friction is what holds it for life. Some sockets also have a hole through the side for a bolt or rivet, so check for that before you buy a handle.

And to whoever asked about the burning method, yeah that's a real concern. Lighter fluid burns hot enough to draw the temper right at the socket, so the steel there can go soft and bend over time. Soaking the old wood in the socket out with some penetrating oil and pulling it out in pieces is slower but keeps the heat off the steel.

One thing worth adding once the head is clean, put a fresh edge on it before you call it done. I rehab a couple of shovels at work every spring, and running a bastard file along the lip at a shallow angle takes me maybe 15 minutes per head. Makes a huge difference in how it bites, especally in clay. Also eyeball the socket taper before you buy the handle, hold the old broken stub next to the new one and check the angles match, a mismatched taper will never seat tight.

Done a few of these handle swaps and the slam method works but here's the thing people skip - don't buy the cheapest handle on the rack. The bargain ones are machined rough and they'll never seat all the way down the taper. Spend a couple bucks more and it'll drop in snug with way less hammering. Also hit the finished handle with some boiled linseed oil before you use it, raw handles dry out and crack within a season outside. Cheap insurance for a tool you just saved.

Agree with the filing advice, a fresh edge is the part most people skip. I did this on a spade I rescued from a rental house last year and clamping the head flat to the bench made the file work way easier than holding it in one hand. I kept checking the angle against the factory bevel on the sides so I didn't overdo it. Took a couple passes with the bastard file, then a few light strokes with a mill file to smooth it out. Dug through compacted gravel the next weekend and it bit like a new tool. Worth the extra ten minutes over just stopping at the rust removal.

Been doing these swaps since the 70s when you couldn't just buy a pre-tapered handle at the store. We'd whittle the tenon down with a drawknife and fit it by eye, checking the seat as we went. Took longer but you learned real quick what a proper fit felt like. One tip from those days, if the socket has the old rivet still in it, drive that stub out with a punch before you try seating anything, or the new handle will never sit flush. Also I never slam these on concrete, a big block of wood on the garage floor absorbs the shock and the grain in the handle stays intact.